gormanghaste from WarcraftPets posted some interesting changes to vanity pets!
many pets that currently do not make any sounds or noises (on-click or periodic) have apparently found their voices on the PTR. if these updates go live, we'll start hearing more from once silent companions such as the sinister squashling, sprite darter hatchling, and tickbird hatchling, just to name a few.
not all pets have had their on-click vocalizations updated, and some companions may still remain quiet once patch 4.1 goes live. i'm pretty eager to hear at least a few on live servers, though!
additionally, rhapture also discovered something pretty neat. if you log out with a vanity pet on the PTR, you will resummon that same pet once you log back into that toon. i just tried it with my premade character, and although i was last online yesterday night, it worked! my druid had a content little nightsaber cub waiting patiently next to my character the moment i logged in. sweet!